Phillip in Halifax, Nova Scotia has painted all his wooden double hung windows shut tight. So how do you open them without damaging them?
Start by cutting the paint at the joint between the window and the frame with a knife.
Then put a block of wood on the window sash, the frame that holds the glass, starting at the middle of the window where the two sliders meet, and give it a sharp whack with a hammer. Not so hard as to break the glass or anything, but enough to shock the paint.
Then as it begins to move a little, move down and around the whole window, breaking it loose.
If it still doesn't want to open, then you need to go to a flat, thin but rigid pry bar. Put a thin piece of wood under the pry bar to protect the window sill and only work on the two sides where there is vertical wood, not glass above the prybar. Then dig into the bottom of the window a bit with the pry bar and try leveraging it up. What you don't want to do is to shove up on the top part of the wood framing the glass -- you might pop this piece of wood right off of the window assembly.